In a vehicle in which an automatic steering control is used, a steering mode is switched between a manual steering mode and an automatic steering mode. In the manual steering mode, a rudder angle of the steered wheel is controlled according to an operating position of a steering input device which is operated by a driver. In contrast, in the automatic steering mode, a target rudder angle of the steered wheel for the automatic steering is calculated, and a rudder angle variable device is controlled so that the rudder angle of the steered wheel becomes the target rudder angle. Then, when a predetermined initiation condition for the automatic steering is satisfied, the steering mode is switched from the manual steering mode to the automatic steering mode, and conversely, when a predetermined termination condition for the automatic steering is satisfied, the steering mode is switched from the automatic steering mode to the manual steering mode.
In particular, when the steering mode is switched from the automatic steering mode to the manual steering mode, a gain of the automatic steering control is decreased from 1 to 0, the gain being a ratio of an actual modification amount of the rudder angle to a modification amount for the target rudder angle of the steered wheel for adjusting the rudder angle of the steered wheel to the target rudder angle for the automatic steering control. By this, the steered wheel becomes steerable according to a driver's steering operation.
Therefore, for example, as disclosed in the following patent document 1, it is known that when the steering mode is switched from the automatic steering mode to the manual steering mode, the gain of the automatic steering control is gradually reduced so that a “steering characteristic” (a characteristic of change of a rudder angle of a steered wheel in response to a steering operation) is not rapidly changed. When the steering mode is switched from the automatic steering mode to the manual steering mode, if the gain of the automatic steering control is gradually reduced, an overriding degree of the manual steering to the automatic steering by the automatic steering control is gradually increased. Therefore, compared to a case in which the gain of the automatic steering control is abruptly reduced to 0, it is possible to reduce unusual feeling which is felt by a driver due to the change in the overriding degree.